Human umbilical cord blood (UCB) has been successfully used as an alternative source of allogeneic hematopoietic stem cells for pediatric transplantation Clinical banking of UCB requires volume reduction and red cell depletion for cost-effective storage. We have compared processing UCB by Ficoll, Percoll, methylcellulose, gelatin, starch, and red cell lysis. As individual UCB collections vary widely in colony forming cell (CFC) and CD34+ cell content, each UCB (n = 26) was processed by three or more techniques in parallel with Ficoll as the 'standard' method. Gelatin gave a consistently high recovery of CFC (92%) and CD34+ cells (86%). Between 0.10-2.50% of the leukocytes in gelatin-treated UCB were CD34+ with an intra-assay variation of 2.1%. Combining data from individual experiments, the correlation between CD34+ and CFC content was excellent (r = 0.77). Lysis rated second in terms of CD34+ and CFC recoveries but is not as practical because of the large volumes involved. Ficoll and Percoll came third but are more expensive and more involved techniques. Starch sedimentation proved to be slow, while methylcellulose processing lost over 60% of CFC and CD34+ cells. After gelatin processing, we calculated 70-mL donations of UCB would contain a mean ± SD of 9 ± 2 x 108 nucleated cells, 32 ± 18 x 105 CD34+ cells, and 20 ± 12 x 105 CFC with greater than 95% red cell depletion. Recent published computer studies suggest that as few as 2 x 105 CD34+ cells maybe needed for sustained engraftment of allogeneic marrow in adult transplant recipients. We conclude that average 70-mL UCB donations contain sufficient marrow repopulating cells for adult recipients.